To achieve mobility in a wireless cellular communication system, continuous service has to be offered even if a user equipment (UE), such as a mobile phone, moves between cells of the mobile communication network. The continuous service is achieved by a handover (or handoff), which is generally initiated either by crossing a cell boundary or by a deterioration in the quality of the signal in the current channel. During the handoff, the link between the user equipment and the mobile communication network needs to be adapted, e.g. by terminating a link to the base station (BS) of the cell which is left and setting up a new link with the base station of the cell which the UE enters. Other than during handoff, link adaptation is generally used in the wireless communication to match the modulation, coding and other signal and protocol parameters to the conditions on the radio link. The adaptation can be performed dynamically so that the signal and protocol parameters change as the radio link conditions change. As an example, the modulation and coding scheme may be adapted to the quality of the radio channel by means of a rate adaptation algorithm to ensure robustness of the data transmission.
To perform an efficient adaptation, some information on the transmission channel is required at the transmitter. To obtain such information, the UE for example measures the signal strength or quality of the neighbour cells during cell selection or reselection and handover. Depending on the type of mobile communication network, the UE or the base station measure different operating parameters. Base station here generally means the node of the wireless communication network communicating with the UE. In a universal mobile telecommunications system (UMTS) network, the UE may for example measure the received signal strength indicator (RSSI), while in a long term evolution (LTE) network, the UE may measure the reference signal received power (RSRP) and the reference signal received quality (RSRQ). The UE can measure such measurement quantities by performing physical layer measurements that are processed by the UE and then conveyed to the base station.
An implementation of such measurements is for example given in the 3GPP reference TS 36.331 V8.6.0, which specifies the radio resource control protocol for the E-UTRAN (evolved UMTS terrestrial radio access network) radio interface. The measurement processing in the UE comprises two stages, namely the physical layer filtering in the linear domain and a higher layer filtering in the logarithmic domain. The physical layer filtering can simply be the averaging of several measurement snapshots. The higher layer filtering is then used for post processing the results from the physical layer filtering. Section 5.5.3.2 of the document mentioned above describes a higher layer filtering using a one tap infinite impulse response (IIR) filter. The latest received measurement result from the physical layer (after physical layer filtering) is filtered by using a fixed filter coefficient which is preset in accordance with the sampling rate. A certain time characteristics of the filter is thus obtained. The higher layer filtered measurement result can then be used for link adaptation.
The problem now raises that in certain situations, physical layer measurements are not available for the higher layer filtering operation. Situations in which physical layer measurements may not be available include the continuous receiving of data by the UE, as a result of which the UE cannot perform the physical layer measurements at the same time; the UE being operated in an idle period; the UE being in a period with discontinuous reception (DRX); and the UE having a measurement gap for measurements from a cell that is different from the cell considered. When such physical layer measurement gaps are present, the results of the higher layer filtering can be misleading and do not correctly reflect the physical layer general conditions for the UE. In particular, the reported values are supposed to provide up to date information about the physical layer channel conditions, but the tracking behaviour of the measurement reports with respect to the time varying channel conditions is deteriorated in case of missing physical layer measurements. Furthermore, the filtering is not transparent for the base station, as it does not have any information about physical layer measurement gaps of the UE, so that it is difficult for the base station to interpret the higher layer filtered measurement results. This may result in an incorrect link adaptation, and consequently in a degradation of the quality of communication between the UE and the base station.
It is thus desirable that the filtering results obtained after higher layer filtering more closely reflect the actual channel conditions. In particular, it is desirable to reduce the influence of measurement gaps on the higher layer filtered measurement result, and to enable a meaningful interpretation of these results.
It is thus an object of the present invention to obviate at least some of the above disadvantages and to provide an improved filtering of measurement values of a measurement quantity.